|Author:||Duquette and Mathieu, 1966|
|Reference section:||Reference outcrop in sheet 32J01-200-0102 (UTM NAD83, zone 18: 561847E, 5541786N) located in a section described by Bélanger (1979) along the Waconichi Lake access road|
|Type area:||NTS sheets 32G09, 32G10, 32G15 and 32G16|
|Geological province:||Superior Province|
|Lithology:||Felsic volcanic rocks|
The Waconichi Formation established by Duquette and Mathieu (1966) originally consisted of volcaniclastic rocks occupying the Waconichi Lake basin, in the NE quarter of McKenzie Township (sheet 32G16-200-0201). The thickness of this unit was initially estimated at 12 500 m, but was reduced to 2400 m following Caty’s mapping of Richardson Township (1975), which limited the Waconichi Formation to volcaniclastic rocks between Gwillim and Waconichi lakes (northern portion of sheet 32G16). Subsequently, several small and spatially discontinuous felsic rock units were incorporated into the Waconichi Formation (Allard, 1976; Henry and Allard, 1979; Allard et al., 1985; Daigneault and Allard, 1990; Chown et al., 1991). Thus, the Lemoine (rhyolitic rocks), Queylus (volcaniclastic rock) and Portage (Algoma-type iron formation and volcaniclastic rock) facies were included in the Waconichi Formation based on petrographic and geochemical criteria (Daigneault and Allard, 1990; Chown et al., 1991).
Detailed mapping of the Haüy area subsequently led to the addition of the Andy (Daigneault and Allard, 1990), Lacs, Coyote, Chevrier and Îles (Legault, 2003) members. Characterization of felsic volcanic rocks following detailed mapping, drilling, geochronology and geochemistry has resulted in significant refinement of the stratigraphy of this unit (Lafrance et al., 2006; Roy et al., 2007; McNicoll et al., 2008; Leclerc et al., 2011; David et al., 2012). The Lemoine, Queylus and Portage facies are now considered formal members of the Waconichi Formation, which also includes the Scott and Allard members (Leclerc et al., 2011). The Lemoine Member contains an informal division that distinguishes the lower and upper portions of the unit (Roy et al., 2007; Boulerice et al., 2013; Mercier-Langevin et al., 2014). The Deux Orignaux Member is introduced to designate Waconichi Formation rocks exposed in the northern part of Deux Orignaux Lake (northern portion of sheet 32G14), while the Winchester Member includes rocks located south of Winchester and Trois Îles lakes (sheet 32G10-200-0102).
The Waconichi Formation is mainly exposed on the north and south flanks of the Chibougamau Anticline (sheets 32G09, 32G10, 32G15, 32G16), near the Chibougamau Pluton and Lac Doré Intrusive Suite.
The Waconichi Formation consists of the following twelve members:
The reference outcrop exposes a green-beige monomictic block tuff cemented by a light to dark grey matrix. South of Waconichi Lake, this facies forms lenses in a sequence of quartzofeldspathic and feldspathic metatuffs (Bélanger, 1979). These volcaniclastic rocks are part of the Waconichi Formation as originally established by Duquette and Mathieu (1966).
The actual thicknesses of the various members of the Waconichi Formation generally do not exceed 1000 m (Daigneault and Allard, 1990; Leclerc et al., 2017). Most of the units associated with the Waconichi Formation appear on the north and south flanks of the Chibougamau Anticline, near or directly in contact with rocks of the Lac Doré Intrusive Suite and Chibougamau Pluton. This is the case for rocks assigned to the Queylus, Andy, Portage, Lemoine, Scott and Allard members (sheets 32G09, 32G10, 32G15, and 32G16). Rocks of the Chevrier, Coyote, Îles, Lacs and Winchester members are distributed between the Verneuil Pluton and Eau-Jaune Complex (sheet 32G10). The Deux Orignaux Member outcrops west of Chapais (northern portion of sheet 32G14).
The emplacement of the Waconichi Formation has been dated at several locations. The following ages of volcanism were obtained:
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||2726.6||0.7||0.7||Leclerc et al., 2011|
|U-Pb||Zircon||2726.7||0.7||0.7||Leclerc et al., 2011|
|U-Pb||Zircon||2727.4||0.9||0.9||Leclerc et al., 2011|
|U-Pb||Zircon||2726.7||1.1||1.1||David et al., in preparation|
|U-Pb||Zircon||2728.2||0.8||0.8||Leclerc et al., 2011|
|U-Pb||Zircon||2728.7||1.0||1.0||Leclerc et al., 2011|
|U-Pb||Zircon||2729.0||1.1||1.1||David et al., 2012|
Rocks of the Waconichi Formation form the upper part of the Roy Group’s first volcanic cycle (Allard et al., 1979; Daigneault and Allard, 1990; Leclerc et al., 2017). The different members of this formation correspond to discontinuous but contemporary felsic volcanic centres overlying a plain of basalts and andesitic basalts (Obatogamau Formation). Rocks of the Waconichi Formation are overlaid by basalts and andesitic basalts of the Bruneau Formation, which characterizes the base of the Roy Group’s second volcanic cycle.
Does not apply.